Communication systems are becoming increasingly important in all aspects of business and personal life. Modern communication systems transmit data at very high rates of speed. The data is typically transmitted as a series of bits represented by high or low voltage levels in a signal, commonly called a bit stream. In these systems the receiver must be able to accurately and reliably decode the bit stream to extract the transmitted data.
One difficulty in extracting transmitted data from a bit stream is determining where each bit resides in the stream. The location of each bit in the bit stream needs to be identified to determine where the receiver should sample the data stream to accurately extract the data. Otherwise the receiver could miss data bits, sample the same data bit twice, misinterpret a data bit, or otherwise corrupt the received data.
This task is made more difficult and critical by the presence of noise in the communication system which can distort the shape of the data stream. Furthermore, the bit stream can commonly include consecutive bits that are either all high or all low, with no transitions between consecutive bits. Even where transitions between bits exist it can be difficult to quickly and accurately determine where the center of each bit resides. The problem of locating bits in a bit stream is commonly referred to as training to find “eye center” of bits in a data stream.
This problem is particularly acute in burst mode communication devices, where individual clients are assigned specific time slices in which they broadcast short bursts of data. In these systems the receiver must be able to accurately determine the location of bits for each burst of data. Typically there are only a few bits of preamble available in each burst upon which this determination can be made, before actual data bits that must be sampled begin to arrive.
When the bit eye center has been located during the preamble there may still be a need to adjust the bit eye center during the remainder of the burst. Specifically, during the burst, the eye center of the data stream can drift from the original located position in the data stream. This can be caused by a variety of factors. For example, small differences in clock speed between the transmitter and the receiver can cause the eye center of bits in the burst to drift over time. When this drift is severe, the sampling point can move unacceptably close to the transitions, causing potential errors in the data sampling and recovery. To avoid these potential errors, it is desirable to adjust the location of the sampling to compensate for any drift that is occurring. This is generally referred to as retraining to find a new bit eye center or sample point. In many systems it is required to retrain to avoid unacceptable data losses that would otherwise occur.